1. Field of the Invention
The present invention relates to the production of catalysts and adsorbents which can be used for the removal of nitrogen oxides from exhaust gases containing the same.
2. Description of the Background
Nitrogen oxides (hereinafter referred to as NO.sub.x) are found in combustion exhaust gases from industrial plants and automobiles and lead to the production of photochemical smog. Methods to prevent the formation of such smog are urgently needed for environmental safety.
At present, both dry and wet methods are known for the removal of NO.sub.x. Using wet methods, the NO.sub.x is contained in exhaust liquids which are quite difficult to treat. Therefore, the wet methods are still not of practical use. Among the dry methods, there are known methods such as non-catalytic reduction, direct catalytic decomposition, selective catalytic reduction, and adsorption. Among these methods, the selective NH.sub.3 catalytic reduction method is already in practical use. However, this process requires NH.sub.3 as a reducing agent, and further requires devices for the recovery or decomposition of unreacted NH.sub.3, thus leading to a more complicated process.
The direct catalytic decomposition of NO.sub.x is the most preferred method, since it entails the simplest process and does not require a reducing agent such as NH.sub.3. Hitherto, a number of investigations have been performed on the direct catalytic decomposition of NO.sub.x. Pt, CuO and Co.sub.3 O.sub.4 are known to exhibit catalytic activity in decomposing NO.sub.x, but the activity remains insufficient due to the poisoning action of the decomposition product oxygen. Thus, these catalysts cannot be used in practice.
Recently, the zeolite (ZSM-5) which contains copper ions and has a specific crystalline structure has been found to act as a decomposition catalyst for NO (Japanese Laid-Open Patent Application No. Sho 60-125250) in the direct catalytic decomposition of NO.sub.x which does not suffer from the poisoning action of moisture and oxygen even if they are present in the gas to be treated. The ZSM-5 containing copper as disclosed in the cited reference is prepared from conventional ZSM-5 by subjecting the same to ion exchange in an aqueous solution of a water-soluble divalent copper salt.
There are a large number of reports regarding ZSM-5containing copper. In Japanese Laid-Open Patent Application No. Sho 54-96500, for example, the ZSM-5 containing copper is prepared from ZSM-5 by repeated ion exchange with an aqueous solution of a watersoluble divalent copper salt. The product is used as a catalytic combustion catalyst.
In Japanese Laid-Open Patent Application No. Sho 57-36015 and U.S. Pat. No. 4,297,328, the ZSM-5 is ion exchanged three times with an aqueous solution of copper(II) chloride at a refluxing temperature to obtain a rate of exchanged copper ions of larger than 160%. The prepared materials are used as a combustion catalyst and as ternary catalyst, respectively.
However, as long as the conventional ion exchange method is employed, the exchange of a large amount of copper is not possible in a single run, instead, several repeated runs of exchange are needed.
ZSM-5 containing copper which evinces activity in the direct catalytic decomposition reaction of NO can be prepared by the process described in the existing literature. However, the ion exchange carried out as described in the literature does not yield a sufficient amount of copper ions in a single run and repeated runs are necessary. Therefore, several repeated procedures of ion exchange are needed to obtain a larger amount of exchanged copper and to attain a high activity for the decomposition of NO.
Thus, a need continues to exist for a process by which nitrogen oxides in exhaust gases can be catalytically decomposed in an efficacious manner. A need also continues to exist for a process by which a catalyst having the above-described properties can be prepared in high yield.